As a server I can say that most others that I’ve known from the industry have had at least one encounter where they were 100% if not almost entirely certain that their table was planning on paying with a stolen credit card. I don’t suggest discriminating, but coming from the industry; when you know, you know. I convinced a table who racked up a $400 bill to tip the bill knowing they didn’t care what they spent on the card. A couple weeks later they were caught in my restaurant. They had to pay back all the charges they had run up including whatever they had tipped. The money never came out of my pocket once they were caught. Easy way to a couple extra hundred bucks in a shift that won’t hurt the victims account once the thief is caught which is more common than not.
Asking if they want change back also makes them rethink whether they actually want to tip you as much. Don't give them that chance.